1. Field of the Invention
This invention relates in general to nuclear reactor fuel assemblies and more particularly to control rod guide thimble designs for use within nuclear reactor fuel assemblies.
2. Related Art
In nuclear reactors of the type designed in the former Soviet Union, the reactor core is comprised of a large number of elongated fuel assemblies, each having a plurality of fuel rods held in an organized hexagonal array by a plurality of grids spaced longitudinally along the fuel rods and secured to stainless steel control rod guide thimbles. The stainless steel control rod guide thimbles extend above and below the ends of the fuel rods and are attached to the top and bottom nozzles, respectively. The fuel assemblies are arranged in the reactor vessel with the bottom nozzles resting on a lower core plate. An upper core plate rests on the top nozzles.
The top nozzles in the Soviet design are non-removably fixed to the stainless steel control rod guide thimbles of the fuel assembly. These complex nozzles perform several functions. First, they position the rod control cluster assembly (RCCA) relative to the control rod guide thimbles within the core so the position of the RCCA relative to the upper core plate is fixed. The RCCA positions the control rods, which are inserted into the fuel assembly as a group or cluster.
The Soviet nozzle also dampens the velocity of the control rods using springs to remove energy when the RCCA rods are dropped into the reactor core during an emergency shutdown, commonly known as a “scram”. The nozzle also supplies spring loads for supporting the internals. When the upper core plate is lowered onto the nozzles, it compresses the nozzle spring. In addition, the Soviet nozzle is designed to protect the control rods when the fuel assembly is removed from the reactor vessel. Under these conditions, the RCCA is at or below the top edge of the nozzle. Finally, the Soviet design of the top nozzle allows the fuel assembly to be handled when lifted out of the core by transferring the loads through the nozzle.
Thus, the Soviet nozzle is designed to function in two positions, free and compressed. As stainless steel is used for the thimbles of the Soviet fuel assembly, the relative separation between the interior of the reactor vessel and the fuel assemblies remains constant once the assembly is in position. Spring loads are such that the nozzles can support the internals, and the spring loads as well as the RCCA positions are fixed so that all functions are static. As a result, the nozzle has built-in references around which the internals are designed. The stainless steel thimbles used in the Soviet design impose higher reactivity cost on the fuel assemblies due to their neutron capture cross-section, i.e., neutron absorption rate, and they are more difficult to attach to the grids of the fuel assemblies. Non-Soviet fuel assemblies utilize zircaloy for the thimbles which imposes less reactivity cost. However, zircaloy has a different coefficient thermal expansion than the stainless steel reactor vessel, and grows during irradiation. Expandable top nozzles, which accommodate for these variations in the dimensions of the different components within the reactor are disclosed in, for example, U.S. Pat. Nos. 4,534,933; 4,687,619; 4,702,882 and 4,986,959. Such nozzles, however, are used in reactors in which the top core plate rests on a core support in the form of a circular ledge within the reactor vessel. In the Soviet type reactor, the core plate rests on and is supported by the top nozzles.
As mentioned, the Soviet design top nozzle is permanently attached to the thimble tubes of the fuel assembly. The above-mentioned patents disclose removable top nozzles and U.S. Pat. No. 5,479,464 took that technology to another step in applying the removable top nozzles to the Soviet type reactor nozzle design. However, the substitution of zircaloy for stainless steel in some of the fuel assembly components, such as the thimble tubes in which the control rods move, requires further modifications to assure that impact loads experienced by the assemblies can be absorbed without damaging the assemblies or other core components. For example, in the VVER 1000 type Soviet designed reactor, when the control rods scram, they freefall and impact the top nozzle at a very high velocity. This fuel design does not use a dashpot or any other hydraulic mechanical device to minimize these high impacts. In the design described in U.S. Pat. No. 5,479,464, springs are employed to absorb some of this load. However, further means are desired to absorb the shock of the load as well as the load itself. In a standard western fuel assembly design, approximately two feet before full insertion of the control rods into the fuel assembly, the tips of the control rods enter a small diameter portion of the thimble tube called the dashpot. This dashpot is approximately one (1) millimeter larger than the control rods. Because the control rods are moving very fast at this point in the scram, there is a large volume of water which has to be accelerated up past the falling control rods to make room for them in the dashpot. This process causes the control rods to decelerate rapidly, thus lessening the impact velocity of the control rod assembly at the top nozzle adapter plate. The standard VVER 1000 style fuel assemblies do not have a dashpot and therefore the control rod assembly impacts the top nozzle at a much higher velocity. As the kinetic energy is equal to one half the mass x the velocity2, if the velocity at impact on the VVER 1000 fuel design is four times that of the standard western pressurized water reactor design, then the total energy which has to be absorbed after impact is sixteen (16) times as much. A new high energy absorption top nozzle has been designed to absorb that energy and is described in U.S. Pat. No. 6,738,447. This high energy absorption top nozzle assures that the impact loads expected during scram events will be absorbed without damaging the nozzle, fuel assembly and/or control rod assembly. In addition, this new design is expandable to accommodate expansion and growth of the zircaloy components of the fuel assembly while supporting the upper core plate in a fixed position. More recently, the Temelin Unit 1 and 2 reactors of the VVER 1000 type design have experienced some problems associated with incomplete control rod insertions, which raises some safety concerns.
Accordingly, there is a further need for an improved VVER 1000 type of fuel assembly design that will overcome the incomplete control rod insertion problem while accommodating zircaloy clad control rod thimble tubes.